Tbf I can kind of see why it could have got to 3 months without a visit due to the circumstances you've outlined. Have you offered to visit them?
They live 3 hours away so they cant just pop in for half an hour for coffee, a trip will take up an entire day at the very least, which is no mean feat at this end of the year.
You say that they both work, so assuming they work weekdays, they're then limited to weekends for visits which immediately reduces availability.
Unless they cleared their diary for a couple of weeks around your due date they may not have been able to travel at short notice for the first couple of weeks as they had plans they didnt want to/couldn't cancel, especially when theres two of them and they may both have plans at differing times. For example MIL might be free one weekend but FIL is out celebrating a friend's birthday; next weekend FIL is free but MIL has tickets to a show with friends; the following weekend MIL is free Saturday but FIL is only free on Sunday etc. Also depending on where they work they may have a heavy workload which reduces free time (eg I work in retail so October to December are our peak sales months so its all hands on deck, even senior management are pulling overtime to keep things running)
Then they have dogs. It would be rude of them to turn up with their dogs, especially when you have a newborn, and if they wanted to make a weekend trip many hotels dont allow animals so they'd have to find dog sitters. Many dog sitters are booked up well in advance unless you want to pay extortianate last minute prices, and add that on top of hotel cost and petrol and they may not be able to afford it.
Furthermore you have to factor in their illness - I would not have wanted anyone to visit my DD as a newborn if they knew they were sick because I would not have wanted her to get ill. If someone turned up with a cold and they had travelled a distance i would feel obliged to let them in but i would not want them holding/touching/kissing etc DD to minimise risk of illness, and who wants to travel 3 hours to visit their newborn grandchild and not get any newborn snuggles?
Added to that, if they're under the weather/tired from work, that 3 hour drive is going to be a chore. A 6 hour round trip is tiring, but doing while under the weather would wipe me out. Additionally if they are using up a whole weeknend to visit they are getting no downtime between work weeks, and at this time of year that can make illnesses worse/drag on longer as youre just run down.
Then as a bonus reason, we've just had Christmas and New Year. That is a notoriously busy time of year so they may have had long standing plans, assumed you would have plans, not wanting to have to travel/pay expensive hotel and dog sitting prices for Christmas etc. They sent presents so you didnt see them at all - did you offer to visit them over Christmas?
Add all those things together and it's easy to see how 3 months could pass without a visit.